Definition - What does Nominal Size mean?
Nominal Size, also known as trade size in manufacturing is the size used for the description in name only. It is expected to not match any of the dimensions of a product. But within realm of the product it may correlate with a great number of the highly standardized dimensions. To apply the Nominal Size across the domains, it is required to understand the size system in both areas. For instance, if someone wishes to select a drill bit to clear the 1/4 inch screw, the person may consult the tables to show a proper size of the drill bit.
Petropedia explains Nominal Size
Nominal Size is the measurement used for naming a component, not necessarily the actual size of the component when milled or finished. The size of timber is generally given in the Nominal Size but the actual size is a bit smaller. For instance, the lumber size dimensions like two by four inches of a board whose completed dimensions are nearly one and half inches by three and half inches. A three and a half inch floppy disk has a standard dimension of 3.54 of inches or 90 mm and can hold 1.44 of megabytes despite of its capacity which is 1,474,560 bytes. There are various dimensions which are required for assuring the proper function of the screw threads but Nominal Size is used to refer its dimensions.